1. Field of the Invention
The invention concerns a method of dissolving either a precipitate that contains essentially calcium carbonte (CaCO.sub.3) or a deposit of that type, especially scale, by means of a Lewis acid in a concentration of 10 to 1000 mmoles/l.
2. Background Information
The loss of carbon dioxide to the atmosphere from water that has a high carbonate hardness and total hardness often results in supersaturation with calcium carbonate and magnesium carbonate (MgCO.sub.3) and hence to more or less rapid precipitation of, mainly CaCO.sub.3, but also of CaCO.sub.3.MgCO.sub.3, MgCO.sub.3, and other impurities. The precipitates can appear as cloudiness and particles, but also frequently in the form of hard deposits and crusts of what is called "scale", that are difficult to remove by mechanical means alone.
Scale typically consists of more than 95% CaCO.sub.3, less than 5% MgCO.sub.3, and traces of phosphate and iron. Characteristic of scale, therefore, is that it contains perponderantly CaCO.sub.3.
More scale is deposited as the temperature of the water rises and the carbonate hardness and total hardness increase. Scale can be detrimental in many ways, for example, by decreasing heat transfer, constricting pipes, encrusting pumps, valves, and heating devices, and diminishing the transparency and appearance of glass, plastic, and metal containers, and can lead to serious system malfunction. A significant example of the type of device affected is home automatic coffee and tea makers, and the periodic elimination of scale from such systems is unavoidable.
Since calcium carbonate is the major constituent of scale, the following discussion will be confined to that substance, although applicable, as well, is to dolomite encrustation, CaCO.sub.3.MgCO.sub.3, and MgCO.sub.3, as well as other carbonates like basic copper carbonate or zinc carbonate.
That CaCO.sub.3 can be more or less rapidly dissolved with acid--a strong inorganic acid like HCl of HNO.sub.3, a medium-strength inorganic acid like H.sub.3 PO.sub.4, a medium-strength to weak organic acid like HCOOH or CH.sub.3 COOH, or a weak organic acid like citric or tartaric acid--is known. Mixtures of various acids like H.sub.3 PO.sub.4 and citric acid are also employed.
The often desirable very rapid dissolution of CaCO.sub.3 can be attained only with strong inorganic acids. The rate of dissolution decreases rapidly with the strength of the acid and remains low even with polybasic acids because usually only one proton, the hydrogen ion (H.sup.+), per molecule of acid, mainly the H.sup.+ of the first dissociation stage specifically, participates in the reaction.
Practical experience and prototype testing have demonstrated that acids only with an initial dissociation constant pK that is lower than 1.5 to 2 will react with CaCO.sub.3 at sufficient and practical speeds. Thus only appropriate concentrations of strong mineral acids, HCl and HNO.sub.3 for example, will result in satisfactory CaCO.sub.3 -dissolution times. Even medium-strength acids with initial dissociation stages in the vicinity of pK 2 to 4, even phosphoric acid, that is, react considerably more slowly or must be employed at unreasonably high excesses, in extreme concentrations in other words. Acids with pK.sub.1 values of between 4 and 5 react very slowly and their rates of dissolution can be accelerated only unsatisfactorily even at high concentrations. Weaker acids with pK.sub.1 values higher than 5, finally, will not react at all at practically exploitable rates. These considerations with respect to acid strength apply not only to initial dissociation stages (where they represent the highest reaction stage of any acid), but to all dissociation stages of multiproton acids.
The acids or mixtures of strong and medium-strength acids practically employed at the state of the art, however, have drawbacks from the aspect of applications technology. Thus, even the medium-strength acids exhibit a high level of corrosion, an acrid and unpleasant odor that irritates the mucous membranes, a toxicity that is physiologically considerable, and an aggressiveness that can irritate the skin and mucous membranes. Such acids are detrimental to the environment and necessitate special safety measures in that they are not completely safe to employ or to securely package. Weak acids exhibit a very low and unsatisfactory dissolution of the CaCO.sub.3, accompanied by the formation of cloudy solutions and by the secondary precipitation of calcium salts that are difficult to dissolve, whereas only part of the acid capacity, 25 to 50% for example, is utilized, making higher concentrations and larger amounts necessary.
Tests of weak and hence low-corrosion organic acids have demonstrated that they react very slowly, to a completely unsatisfactory extent, and often accompanied by the formation of cloudy solutions and secondary precipitations that include granules of CaCO.sub.3. The considerably more reactive strong acids exhibit the drawbacks mentioned with respect to medium-strength acids to an even greater extent. Even tests of weak cationic aquo acids, [Al(H.sub.2 O).sub.6 ].sup.3+ and [Fe(H.sub.2 O).sub.6 ].sup.3+, for example, produce only slow reactions. Other cationic aquo acids with 3-valence or 4-valence cations also react only minimally with CaCO.sub.3 or scale, and other Lewis acids perform just as poorly.
The use of aluminum-chloride solutions to dissolve deposits of lime is known from Thermal Engineering 29, 504-05 (1982). The rate of dissolution was essentially lower than that of hydrochloric acid, although higher than that of an organic, aliphatic, low-molecular acid. The corrosiveness of the aluminum chloride was lower than that of the organic acids.